Jack Rafter
A short rafter that runs from a hip or valley rafter to a wall plate or ridge.
Quick Answer
A jack rafter is a shortened rafter that does not span the full distance from wall to ridge. A hip jack runs from a wall plate up to a hip rafter, a valley jack runs from a valley rafter up to the ridge, and a cripple jack touches neither. Estimators count them in roof framing, usually by the piece.
The Full Picture
Most rafters on a simple roof are common rafters, running from the wall plate to the ridge. At a hip or a valley, the roof planes meet at an angle, and the rafters that fill in that plane get shorter step by step. Those shorter pieces are jack rafters. A hip jack frames from the wall plate to the hip rafter, a valley jack from the valley rafter to the ridge, and a cripple jack is cut between a hip and a valley without reaching the plate or the ridge.
Jack rafters sit in the roof framing next to the hip or valley, and the framing plan, roof plan and sections show them. In a truss roof, hips and valleys are formed with truss sets and hip jacks, so the framing plan may mark them as truss pieces instead. For a stick-framed roof, the estimator reads the framing plan and the roof pitch to find the layout. Hip and valley rafters, and the ridge, are taken off with the same section, because the jacks depend on them.
The unit is each (EA), by length, or linear feet if the lumber is priced that way. Jack lengths step down by a common amount: the spacing along the plate times the length-per-foot factor of the slope. For a 6 in 12 pitch the rafter length per foot of run is the square root of (12 squared plus 6 squared) divided by 12, about 1.118, so the common difference for jacks at 24 in on center is 2 ft x 1.118, about 2.236 ft. Overhang and plumb cuts add length, and waste depends on how many jacks can be cut from stock.
Pricing mistakes include counting jacks on only one side of a hip, forgetting that the hip rafter and the ridge board are separate members, and ignoring the longer lumber for the plumb cut. A second mistake is applying the plan length instead of the sloped length. Jacks also take more labor per foot than common rafters because each piece has a compound cut, which a simple LF price may not reflect.
Real Examples
Common Misconceptions
People assume: A jack rafter is a type of common rafter.
Actually: Common rafters run from plate to ridge. Jacks are shorter and framed into a hip or valley, so they have compound cuts and a different length at each position.
People assume: All jacks on a roof are the same length.
Actually: They step by a constant amount along the hip or valley, so lengths vary at every position.
People assume: Hip framing is counted on the plan alone.
Actually: The slope sets the length, so the pitch from the roof plan or section is needed to find the sloped length of each member.
Does MeltPlan Solve This?
Partially — adjacentRoof outlines, pitch and framing layouts are read from the roof and framing plans, and MeltPlan's takeoff service applies to measuring from them: AI pulls quantities from plans and US-based experienced estimators review and adjust them. Cut lists, overhang allowances and truss versus stick-built decisions stay with your estimator.
Get roof framing quantities taken off your plans →Frequently Asked Questions
What is a hip jack rafter?
It is a short rafter that runs from the wall plate up to a hip rafter. Its top end is cut to fit against the side of the hip, and its length depends on its distance from the corner.
What is a valley jack?
A valley jack runs from a valley rafter up to the ridge. It fills the roof plane beside the valley and gets longer as it moves away from the valley's low end.
What is a cripple jack rafter?
A cripple jack is a short rafter between a hip and a valley, so it reaches neither the plate nor the ridge.
How are jack rafters counted?
By the piece for each hip and valley, using the spacing, the roof pitch and the framing plan. Many estimators add the lengths and carry the total in linear feet, with a waste allowance that varies by layout.